Students
understand the concept of plate tectonics including the evidence that
supports it (structural, geophysical and paleontological evidence).
E/S

Why is the Earth so restless? What causes the ground to shake violently,
volcanoes to erupt with explosive force, and great mountain ranges
to rise to impressive heights? The answers to these questions were
discovered as one of sciences’ most revolutionary and recent
theories took shape. It began with Alfred Wegener…

Continental Drift
Alfred Wegener (German Astronomer and Meteorologist) proposed in 1914
that all landmasses were at one time connected as a supercontinent
approximately 200 million years ago which he called Pangaea. In Wegener’s
theory of continental drift, Pangaea progressively split up as the
continents detached themselves and “drifted” away. Wegener
provided physical, fossil, geological, and climate evidence to support
this theory;

• Fit of the continents

Wegener noted that the shape of the continent’s coastlines
appeared to match like pieces in a jigsaw puzzle.

• Fossil evidence (Mesosaurus, Lystrosaurus, Glossopteris)

Noted the occurrence of plant and animal fossils found on the match
coastlines of South America and Africa. (Figure 1)

• Rock type and structural correlations
• Similar age, structure, and rock types on continents on opposite
sides of the Atlantic Ocean. i.e. Appalachian Mountains (North America)
and mountains in Scotland and Scandinavia

• Paleoclimatic evidence
• Found dramatic climate changes on some continents. Most notable
was the discovery of coal deposits (made of tropical plants) in Antarctica
which led Wegener to conclude that this frozen continent in an earlier
time in Earth’s history must have been positioned closer to
the equator – where the milder climate allowed lush, swampy
vegetation to grow. (Figure 2)

The main reason Wegener’s hypothesis was not accepted was because
he suggested no mechanism for moving the continents. His belief that
the force of Earth’s spin (rotation) was enough to cause the
continents to move was not shared by the geologists of the time who
knew that rocks were too strong for this to be true.

Sea Floor Spreading
Harry Hess (geologist and Navy submarine commander during WWII) studied
newly published maps of the seafloor topography indicating the existence
of a world-wide mid-ocean ridge system. He proposed, in the 1960’s,
that ridges are where new seafloor is created from upwelling in the
mantle. It was possible, he stated, that molten magma from beneath
the earth’s crust could ooze up between plates and as this hot
magma cooled, it would expand and push on either side of it. He also
proposed subduction as a mechanism for recycling of the seafloor.
His theory provided a mechanism for continental movement that Wegener’s
model was lacking.

• Samples of the deep ocean floor show that basaltic oceanic
crust become progressively older as one moves away from the mid-ocean
ridge. (Figure 3)

• The rock making up the ocean floor is considerably younger
than the continents – no rock samples older than 200 million
years are found in the ocean crust while ages in excess of 3 billion
years can be found in continental rocks.

• Paleomagnetism studies of the ocean floor demonstrate that
the orientation of the Earth’s magnetic field has changed over
time. (Figure 4)

Figure 4:
A theoretical model of the formation of magnetic striping. New
oceanic crust forming continuously at the crest of the mid-ocean
ridge, cools and becomes increasingly older as it moves away
from the ridge crest with the spreading of the seafloor: a.
the spreading ridge about 5 million years ago; b. about 2 to
3 million years ago; and c. present-day (from http://pubs.usgs.gov/gip/dynamic/developing.html).

Plate Tectonics
Since the early 1960s, the emergence of the theory of plate tectonics
started a revolution in the Earth Sciences. The theory has revolutionized
our understanding of the dynamic planet upon which we live. Unifying
the study of Earth, the theory has drawn together the many branches
of earth science, from paleontology (the study of fossils) to seismology
(the study of earthquakes) to volcanism and mountain building. It
provides explanations as to why earthquakes and volcanic eruptions
occur in very specific areas around the world, and how and why great
mountain ranges like the Alps and Himalayas formed.

The theory of plate tectonics states that the Earth’s rigid
outermost layer (lithosphere) is fragmented into seven major plates
and over a dozen smaller plates that are moving relative to one another
as they ride atop the hotter, more mobile material of the asthenosphere
(Figure 5 and 6). The primary force responsible for the movement of
the plates is heat transfer which sets up convection currents within
the upper mantle.

The boundary between these lithospheric plates is where most of the
action (earthquakes) takes place. Three primary plate boundaries exist
(Figure 6);

• Divergent boundaries – where new crust is created as
the plates pull away from each other (mid-ocean ridge)
• Convergent boundaries – where crust is recycled back
into the mantle
• Transform boundary – where plates slide horizontally
past one another

Figure
6: Artist's cross section illustrating the main types of plate
boundaries (see text); East African Rift Zone is a good example
of a continental rift zone. (Cross section by José F.
Vigil from This Dynamic Planet -- a wall map produced jointly
by the U.S. Geological Survey, the Smithsonian Institution,
and the U.S. Naval Research Laboratory.) (from http://pubs.usgs.gov/gip/dynamic/Vigil.html)

Earthquakes
Hold a wooden pencil at its ends and push up with your thumbs in the
middle. The pencil will bend with little stress placed upon it. However,
apply too much stress and the pencil snaps – rapidly releasing
its stored energy. The rocks of the lithosphere act in a similar manner
to the pencil. Due to relative plate motion, rocks of the lithosphere
are under considerable stress. An earthquake is a phenomenon that
results from the sudden release of stored energy in the Earth’s
crust that generates seismic waves. The boundaries between Earth’s
plates are where earthquake (and volcano) occurrences are concentrated
(Figure 7).

Each and every earthquake generates Primary (P-wave) and Secondary
(S-wave) seismic waves. P-waves are compression or longitudinal waves
that travel the fastest of all seismic waves. P-waves travel through
solids, liquids, and gases. S-waves are shear or transverse waves
which travel slower and pass through solids only.

Students understand the concept of plate tectonics including the
evidence that supports it (structural, geophysical and paleontological
evidence). E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly believe that the continents
randomly drift about the Earth or that the continents are no longer
moving.

Continental Drift, the supercontinent Pangaea, and plate tectonics
are likely terms with which students are familiar, however the idea
that continents are still on the move today offers a challenge to
students and adults. Considering the two timescales involved - human
timescale (say 10,000 years of civilization) of observation is far
too limited compared to the processes of plate tectonic occurring
on a geologic timescale (tens and hundreds of millions of years).

2. Students incorrectly believe that California will
split apart from the rest of the United States and become an island
(or fall into the Pacific Ocean), leaving parts of Southern Nevada
with oceanfront property.

The San Andreas Fault is a transform plate boundary that exists between
the North American Plate and the Pacific Plate. This means that the
land west of the San Andreas Fault is sliding northwest past the rest
of the United States, towards San Francisco. This sliding does not
create any space between the two plates for water to fill in, in fact
the two plates are actually being pushed together by the two plate’s
relative motions as they slide horizontally by each other (VERY slowly).

3. Students incorrectly believe that Earth’s
crust is several 100’s of kilometers thick.

Earth’s crust consists of two types; continental and oceanic.
The less dense continental crust is the thickest, having an average
thickness of approximately 30 km while the much thinner and more dense
oceanic crust has an average thickness of approximately 5 km. In fact,
Earth’s crust occupies less than 1% of Earth’s total volume
and represents the extent to which the deepest wells drilled have
not exceeded.

Which part of Earth’s interior is inferred to have convection currents that cause tectonic plates to move?

Rigid mantle

Asthenosphere

Outer core

Inner core

The movement of tectonic plates is inferred by many scientists to be driven by

tidal motions in the hydrosphere.

density differences in the troposphere.

convection currents in the asthenosphere.

olidification in the lithosphere.

Depth of Knowledge Level 2

The cross section below shows the direction of movement of an oceanic plate over a mantle hot spot, resulting in the formation of a chain of volcanoes labeled A, B, C, and D. The geologic age of volcano C is shown.

The cross section below shows the direction of movement of an oceanic plate over a mantle hot spot, resulting in the formation of a chain of volcanoes labeled A, B, C, and D. The geologic age of volcano C is shown.

Which information indicates that new seafloor rock is forming along the mid-ocean ridge and then moving horizontally away from the ridge?

Most volcanoes are located under ocean water and found near the continental shelves.

Paleomagnetic studies of the ocean floor demonstrate that the orientation of Earth’s magnetic field has remained constant.

Fossils of marine organisms can be found at high elevations on continents.

The age of the seafloor rock increases as the distance from the mid-ocean ridge increases.

Base your answer to the question on the cross section below, which shows an underwater mountain range in the Atlantic Ocean. The oceanic bedrock is composed mainly of basalt. Points X and Y are locations in the bedrock that have been diverging at the same rate. The movement of the North American Plate and Eurasian Plate is shown by the two arrows.

Which statements best describe the age and magnetic orientation of the basalts found at locations X and Y?

The basalt at location X is younger than the basalt at location Y. Both locations have the same magnetic orientation.

The basalts at location X and Y are the same age. Both locations have the same magnetic orientation.

The basalt at location X and Y are the same age. Location X has normal magnetic orientation and location Y has reversed magnetic orientation.

The basalt at location X is older than the basalt at location Y. Location X has reversed magnetic orientation and location Y has normal magnetic orientation.

Depth of Knowledge Level 2

Base your answers to question on the map below. The map shows the continents of Africa and South America, the ocean between them, and the ocean ridge and transform faults. Locations A and D are on the continents. Locations B and C are on the ocean floor.

Which graph best shows the relative age of the ocean-floor bedrock from location B to location C

Graph 1

Graph 2

Graph 3

Graph 4

The diagram below represents the pattern of normal and reversed magnetic polarity and the relative age of the igneous bedrock composing the ocean floor on the east side of the Mid-Atlantic Ridge. The magnetic polarity of the bedrock on the west side of the ridge has been deliberately left blank.

Base your answers to the question on the map below, which shows the location of mid-ocean ridges and the age of some oceanic bedrock near these ridges. Letters A through D are locations on the surface of the ocean floor.

Oceanic bedrock on either side of a mid-ocean ridge is supporting evidence that at the ridges, tectonic plates are

diverging.

converging

locked into place

being subducted

Base your answers to the question on the diagram below, which shows details of a section of a rift valley in the center of a mid-ocean ridge. The vertical lines in the diagram represent faults and fractures within the ocean floor bedrock.

Students understand the concept of plate tectonics
including the evidence that supports it (structural, geophysical and
paleontological evidence). E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources
that will facilitate student understanding of this benchmark.

1. This Dynamic Earth: The Story of Plate
Tectonics
by W. Jacquelyn Kious and Robert I. Tilling. An outstanding resource
covering the historical perspective, development the theory, understanding
plate motion, and plate tectonics and people. It is available as an
online edition and as a downloadable PDF edition (77 pages, 3.7MB)
at http://pubs.usgs.gov/gip/dynamic/dynamic.html

2. Plot that Quake!
The goal of this exercise is to motivate students to question why
earthquakes occur where they do. Students plot earthquake data over
time in order to discover that a pattern develops in the occurrence
of earthquakes worldwide.http://seismo.berkeley.edu/istat/ex_quake_plot/

3. Berkeley Seismological Laboratory
This site is a great starting place which contains a worldwide earthquake
catalog, along with links to United States seismic data, and a map
of California and Nevada earthquakes. http://seismo.berkeley.edu/faq/catalog_0.html

5. IRIS (Incorporated Research Institutions
for Seismology
The IRIS website contains a host of lesson plans and resources for
educators along with earthquake maps, lists, and interactive software.
To access IRIS
Education and Outreach, go to http://www.iris.washington.edu/about/ENO/index.htm

7. Earthquakes/tectonics
This site is a great educational resource, which provides students
with numerous links for studying continental drift, plate tectonics,
the structure of the Earth, earthquakes, and seismic waves. The following
is a link to animations, simulations and additional teaching resources
within the science of geology. http://www.scienceman.com/pgs/00_links_geology.html